The present invention relates to a method and apparatus for measuring, with extremely high precision, a predetermined parameter, such as displacements (e.g., physical movements or deformations) of or in a body, temperature of a body, strain in a body, and many other characteristics or conditions influencing, in a known or predeterminable manner, the transit time of energy (electromagnetic or sonic) from an energy transmitter to a receiver.
As briefly described in the above-cited International Applications, systems involving the detection and/or measurement of body displacements are used in a wide variety of medical fields. One important use is as an apnea detector for detecting cessation of breathing particularly of infants, to prevent SIDS (Sudden Infant Death Syndrome), as described for example in U.S. Pat. No. 6,150,941. Another use is for preventing bedsores (decubitus ulcers). Further applications are for monitoring irregular breathing while sleeping, or for monitoring various cardiovascular conditions such as pulse rate, blood pressure, cardiac output and the like. For example, U.S. Pat. No. 5,853,005 illustrates an acoustic monitoring system for monitoring many different type of body functions based on the use of a sensor pad for sensing acoustic signals originating from the body being monitored.
The above-cited International Application PCT/IL00/00241 describes a method, and also apparatus and a probe for use in such method, of measuring a predetermined parameter having a known relation to the transit time of movement of an energy wave through a medium, by: transmitting through the medium a cyclically-repeating energy wave; receiving the cyclically-repeating energy wave transmitted through the medium; detecting a predetermined fiducial point in the received cyclically-repeating energy wave; continuously changing the frequency of transmission of the cyclically-repeating energy wave in accordance with the detected fiducial point of each received cyclically-repeating energy wave such that the number of waves received is a whole integer; and utilizing the change in frequency to produce a measurement of the predetermined parameter. That application describes many uses of such a method, as well as of apparatus and a probe for implementing the method, in many fields, both medical and non-medical, for providing measurements having a much higher degree of precision than otherwise practically attainable.
The above-cited International Application PCT/IL02/00854 describes such a method in which the transmitted and received cyclically-repeating energy wave is an electromagnetic carrier wave amplitude-modulated by a cyclically-repeating modulating wave; the received amplitude-modulated carrier wave being demodulated, and the fiducial point of the demodulated wave being utilized to change the frequency of the modulating wave such that the number of received demodulated waves is a whole integer. Such a method enables the use of high frequency, compact, narrow-beamed antennas or optical systems for transmission and reception.
International Application PCT/IL02/00854 also describes a method and apparatus of making measurements according to the above-cited PCT/IL00/00241 wherein the phase of the received cyclically-repeating energy wave is shifted by a whole-integer multiple of 360° before being utilized to change the frequency the energy wave is transmitted through the medium. This feature adds an artificial distance to the measurement, e.g., when a relatively high frequency is used and therefore a relatively small wavelength is involved, or when otherwise there is a relatively short transit distance between the transmitter and the receiver.